1. Field of the Invention
The present invention relates to a microorganism useful in the control of fire blight, a disease that damages pome fruit trees. More particularly, the invention relates to a bacterium identified as a strain of Erwinia herbicola; methods of using the strain to control fire blight; and agricultural compositions which incorporate the bacterium, which are useful in such methods.
2. Description of the Art
Fire blight disease, caused by the bacterium Erwinia amylovora, can be a devastating disease of pome fruit trees (Malus, Pyrus), and is a major constraint to pome fruit production in many areas of the world. Fire blight is most commonly initiated by epiphytic populations of Erwinia amylovora that develop on blossoms. Under relatively dry climatic conditions, it is thought that bacterial colonization occurs predominantly on flower stigmas, and subsequent rain or heavy dew facilitates movement to the hypanthia where infection generally occurs. Infected blossoms first appear water soaked; then they wilt, shrivel, and turn brown to black. Progression of the infection leads to similar symptoms on other parts of the tree. The most obvious symptom on pear or apple trees is the scorched appearance of leaves on affected branches. Although in the Pacific Northwest, fire blight outbreaks are episodic and only occasionally serious, in temperate climates with high seasonal precipitation, the disease can be production limiting. Pears are no longer commercially produced on the East coast because of this disease.
Considerable expense is incurred by the tree fruit industry each year for labor and chemical costs for fire blight management, as well as to meet stringent export protocols required by some importing countries to minimize the likelihood of pathogen spread. Current control measures rely on a combination of predictive modeling of infection periods, antibiotic spray application, and cultural controls (pruning and sanitation). While control programs have focused on the suppression of Erwinia amylovora on floral parts through the use of antibiotics, the usefulness of this approach has decreased because pathogen resistance to streptomycin has developed in many production areas, and the less effective antibiotic oxytetracycline has been substituted.
Microbial biocontrol of the blossom blight phase of fire blight has been proposed as an alternative to antibiotics (Beer et al., Acta Horticulturae 151:195-201 (1984); Epton et al., In: Ecology of Plant Pathogens, (Eds.) J. P. Blakeman and B. Williamson, pages 335-352, CAB International, Wallingford (1994); Johnson et al., Phytopathology 83:478-484 and 995-1002 (1993); Vanneste et al., Acta Horticulturae 273:409-410 (1990) and 411:351-353 (1996); and Wilson et al., Phytopathology 83:117-123 (1993)); or as a measure that could complement the use of antibiotics (Lindow et al., Phytopathology 86:841-848 (1996) and Stockwell et al., Phytopathology 86:834-840 (1996)). The strains most often referred to in published studies on biological control of fire blight include Pseudomonas fluorescens strain A506, Erwinia herbicola strain C9-1, and Erwinia herbicola strain Eh252. U.S. Pat. No. 4,569,841 to Liu reports Erwinia herbicola strain EHO-10 as having broad spectrum inhibitory activity against pathogenic bacteria including Erwinia amylovora, in in vitro studies. Recently, the product Blightban.RTM. (active ingredient, Pseudomonas fluorescens strain A506) produced by Plant Health Technologies (Boise, Id.) became available commercially.